<!DOCTYPE html>
<html lang='en'>

<head>
  <meta charset='UTF-8'>
  <meta name='viewport' content='width=device-width, initial-scale=1.0'>
  <title></title>
  <script type='text/javascript' src='js/vue.js'></script>
</head>

<body>
  <div id='app'>
    <div>
      <input type="text" v-model="a"> 
      <select name="" id="" v-model="ope">
        <option value="+">+</option>
        <option value="-">-</option>
        <option value="*">*</option>
        <option value="/">/</option>
      </select>
      <input type="text" v-model="b"> 
      <input type="button" value="add" @click="add"> 
      <input type="text" v-model="c">
    </div>
  </div>
  <script type='text/javascript'>
    var vm = new Vue({
      el: '#app',
      data: {
        a: 0,
        b: 0,
        c: 0,
        ope: '+'
      },
      methods: {
        add() {
          this.c = eval(Number(this.a) + this.ope +  Number(this.b));
        }
      },
    });
  </script>
</body>

</html>